1. Field of the Invention
The present invention relates to a scanning electron microscope capable of imaging a specimen area wider than an observable area determined by the minimum magnification of the microscope.
2. Description of the Related Art
A scanning electron microscope has an electron gun for producing an electron beam that is sharply focused onto a specimen to be observed. The beam is scanned across the specimen. As a result, secondary electrons and other electrons are produced from the specimen. A specimen image is displayed on the viewing screen of a display device according to the secondary electrons or other electrons.
In such a scanning electron microscope, the magnification can be varied, for example, from a minimum magnification (e.g., tens of times) to a maximum magnification (e.g., hundreds of thousands of times) by varying the size of the region scanned by the electron beam. For instance, where the width of a viewing screen on which an image is displayed is 200 mm, the electron beam is scanned over a distance of 4 mm across the specimen at a magnification of 50xc3x97. The beam is scanned over a distance of 1 xcexcm at a magnification of 200,000xc3x97.
In a normal imaging method, a wide area of a specimen is imaged at a low magnification to find a portion of interest. Then, a detailed observation is made at this location at a higher magnification. It is convenient to lower the magnification once and to observe a wider area in searching for a portion of interest. Therefore, it is desired to make the minimum magnification as low as possible. To lower the magnification, it is necessary to increase the raster scanning width of the electron beam. If the raster scanning width of the beam is increased, the effect of deflection distortion on the final image increases. Therefore, in a normal scanning electron microscope, the maximum raster scanning width is limited to approximately 5 mm, for example, at which the degree of deflection distortion is tolerable. A correspondingly obtained minimum magnification is about 20xc3x97 to 30xc3x97.
It is an object of the present invention to provide a novel scanning electron microscope capable of imaging a wide specimen area at a magnification lower than a minimum magnification normally dictated by the raster scanning width of the electron beam.
A first embodiment of the present invention provides a scanning electron microscope having a device for directing a sharply focused electron beam onto a specimen, a device for scanning the electron beam across the specimen in two dimensions, an image memory for storing data about an image of the specimen obtained by the scanning, a means for displaying the image of the specimen according to the data stored in the image memory, and a motor-driven specimen-moving device, the scanning electron microscope being characterized in that it further includes a specimen motion control means, an image-synthesizing circuit, a specifying device, and a partial image display. An image acceptance region is established within the area scanned by the beam. The specimen motion control device virtually partitions an area which is scanned by the electron beam and from which an image should be taken into cells defined by grid lines and can scan these cells individually by the electron beam within the image acceptance region. The image-synthesizing circuit stores image data obtained from these cells in locations of the image memory that are addressed corresponding to the cells, thus creating data about a synthesized image. The specifying device permits a human operator to specify an arbitrary area in the synthesized image displayed on the display according to the data about the synthesized image. The partial image display reads image data about the specified area in the synthesized image from the image memory and displays it as a partial image.
A scanning electron microscope in accordance with a second embodiment of the present invention comprises a device for directing a sharply focused electron beam onto a specimen, a device for scanning the electron beam across the specimen in two dimensions, a charged-particle detector for detecting charged particles produced by the scanning, an X-ray detector for detecting characteristic X-rays produced by the scanning, a first image memory for storing image data derived from the charged-particle detector by the scanning of the electron beam, a second image memory for storing data about an X-ray map of a certain element based on an output signal produced from the X-ray detector by the scanning of the electron beam, a first display for displaying image data stored in the first image memory and/or X-ray map data stored in the second image memory as an observable still image, a motor-driven specimen-moving device, a specimen motion control device, an image-synthesizing circuit, and a second display. An image acceptance region is established within the area scanned by the beam. The specimen motion control means virtually partitions an area which is scanned by the electron beam and from which an image should be taken into cells defined by grid lines and can scan these cells individually by the electron beam within the image acceptance region. The image-synthesizing circuit stores image data obtained from these cells by the charged-particle detector in locations of the first image memory that are addressed corresponding to the cells, thus creating data about a synthesized image. The image-synthesizing circuit also stores map data about a certain element obtained from these cells by the X-ray detector in locations of the second image memory that are addressed corresponding to the cells, thus creating data about a synthesized map. The data about the synthesized image and/or the data about the synthesized map are supplied to the second display.
Other objects and features of the invention will appear in the course of the description thereof, which follows.